The present invention is concerned with a method and apparatus for controlling an electric power assisted steering system in a motor vehicle and of the type which uses an adaptive blending torque filter to improve steering feel.
Electric assist steering systems are well known in the art. Electric power assist steering systems that utilize a rack and pinion gear set provide power assist by using an electric motor to either (1) apply rotary force to a steering shaft connected to a pinion gear, or (2) apply linear force to a steering member having the rack teeth thereon. The electric motor in such systems is typically controlled in response to (1) a driver""s applied torque to the vehicle steering wheel, and (2) sensed vehicle speed.
Known electric assist steering systems have a dynamic performance characteristic, known as the system bandwidth, that varies as a function of vehicle speed. As the vehicle operator applies steering torque and rotates the steering wheel back and forth, e.g. left-to-right-to-left, the electric assist motor is energized to provide steering assist commensurate with the steering inputs. How the steering system responds to a particular frequency of back-and-forth steering wheel movement is indicative of the system""s dynamic performance.
The amount of local change at the electric assist motor divided by the amount of local change in steering torque applied by the driver is the steering system gain. A time delay occurs from the time steering torque is applied to the steering wheel to the time the assist motor responds. This time delay is a friction of the frequency at which the input command is applied. This is referred to as the system response time. The system gain is set to a predetermined value so as to have a short system response time while still maintaining overall system stability. The system response time and system gain determine the system bandwidth.
The bandwidth in known steering systems varies as a function of vehicle speed. If dynamic steering frequency or the xe2x80x9cfrequencyxe2x80x9d of a transient response exceeds the system bandwidth at a particular vehicle speed, the steering feel becomes xe2x80x9csluggishxe2x80x9d (felt as a xe2x80x9chesitationxe2x80x9dwhen the steering wheel direction is changed) since the steering assist motor does not respond quick enough. Typically, steering system gain as well as system bandwidth decreases as the vehicle speed increases so that system hesitation or sluggishness becomes more noticeable as vehicle speed increases.
According to our earlier U.S. Pat. No. 5,504,403, which is incorporated by reference herein, it is known for the power assist apparatus to comprise torque sensing means operatively connected to a vehicle hand wheel for providing a torque signal indicative of applied steering torque. Blending filter means are connected to the torque sensing means for providing a blended filtered torque signal having a first functional characteristic at torque frequencies less than a blending frequency and a second functional characteristic at torque frequencies greater than the blending frequency. The apparatus further includes steering assist means for providing steering assist in response to a control signal, and control means operatively connected to the blending filter means for providing said control signal to the steering assist means in response to the blended filtered torque signal. The blending filtering means filters the torque signal so as to maintain a selectable system bandwidth during system operation.
Similarly, according to a later U.S. Pat. No. 5,704,446, which is incorporated by reference herein, a blending filter is provided for splitting the driver torque signal into a low frequency component and a high frequency component. However a further feature is provided where the low frequency component sets the high frequency gain. This is so that the gain for the high frequency component signal mirrors the effective gain of the low frequency component signal, whereby the high frequency signal is close to but always slightly higher than the low frequency signal. As this makes the high frequency gain a function of the amplitude of the low frequency torque signal component, a phase lag is introduced into the high frequency component above the blending frequency which itself could induce instability within the system.
It is an object of the present invention to provide blended filtering of the high and low frequency torques present within the steering system that ensures stability and improved driver feel under all operating conditions.
It is a feature of the present invention to seek to further optimize the torque blending operation in the aforegoing systems.
In accordance with the present invention, there is provided an apparatus for controlling an electric steering assist system, which system provides assist in response to a steering control signal, the apparatus comprising: torque sensing means operatively connected to a vehicle hand wheel for providing a torque signal indicative of applied steering torque, blending filter means connected to the torque sensing means for providing a blended filtered torque signal having a first functional characteristic at torque frequencies less than a blending frequency and a second functional characteristic at torque frequencies greater than the blending frequency, the blending filter means includes a low pass filter which passes all frequencies below the blending frequency and a high pass filter which passes all frequencies above the blending frequency, steering assist means for providing steering assist in response to a control signal, control means operatively connected to the blending filter means for providing said control signal to the steering assist means in response to the blended filtered torque signal, and means whereby high frequency gain of the steering assist system is arranged to be low for on-centre operation of the hand wheel and relatively higher for off-centre operation.
By xe2x80x9con-centrexe2x80x9d operation, we mean operation of the steering wheel in operating regions within a few degrees, typically 5-10xc2x0, deviation from the straight ahead position and where the driver input torques are relatively small, eg. 1 or 2 Nm (see FIG. 14). xe2x80x9cOff-centrexe2x80x9d operation means operation of the steering wheel for larger angles and larger driver input torques, for example, 3 to 4 Nm.
In one embodiment, this operation is achieved in that said means enable the gain of the high frequency path via the high pass filter to be adjusted in accordance with the sensed level of applied steering torque so as to be low for on-centre operation of the steering wheel and relatively higher for off-centre operation.
In another embodiment, an additional frequency dependent transfer function is provided in the blending filter means by which the steering system gain is made to vary as a function of the lower frequency filtered component
Advantageously, the additional frequency dependent transfer function element comprises a proportional plus differential filter.
In some embodiments, the proportional plus differential filter is positioned after an assist curve means which provides a desired steering torque assist signal having a value functionally related to the low pass applied steering torque and sensed vehicle speed. In other embodiments, the proportional plus differential filter may be positioned in front of the assist curve means. The preferred implementation includes the PD filter in front of the assist curve by using a single gain to couple some of the high frequency output from the blending filters into the low frequency path.
The proportional plus differential filter can be realized by a combination of the low frequency and high frequency components (TSL and TSH wherein the input to the assist curve means is given by       T          S      ⁢              xe2x80x83            ⁢      L        +                    ω        b                    ω        z              ⁢          T              S        ⁢                  xe2x80x83                ⁢        H            
where TSL=The low passed signal
TSHthe high passed signal
xcfx89b=blending filter frequency       ω    z    =                              ω          b                ⁢                  G          b                    ⁢              |                  off          -          centre                                    G        ∞            ⁢              |                  off          -          centre                    ⁢              -                  G          ∞                    ⁢              |                  on          -          centre                    
G∞=high frequency gain
Gb=assist curve (low frequency) gain
Advantageously, the transfer function K(s) of the proportional plus differential filter is of the form:             K      ⁡              (        s        )              =          1      +                        1                      ω            z                          ·        s                        w      ⁢              xe2x80x83            ⁢      h      ⁢              xe2x80x83            ⁢      e      ⁢              xe2x80x83            ⁢      r      ⁢              xe2x80x83            ⁢      e      ⁢              xe2x80x83            ⁢              ω        z              =                                        ω            b                    ⁢                      G            b                          ⁢                  |                      off            -            centre                                                G          ∞                ⁢                  |                      off            -            centre                          ⁢                  -                      G            ∞                          ⁢                  |                      on            -            centre                              
with |on-centreand |off-centrerepresenting preferred xe2x80x9cidealxe2x80x9d values and with xcfx89Zxcfx89bGb and G∞as defined hereinafter.
In the present blending filter design, both the low frequency component and the high frequency component set the high frequency gain. This ensures that the high frequency on-centre, and the high frequency off-centre gains can be set uniquely and independently. As the high frequency gain is a function of the low frequency gain, no phase lag is necessarily introduced above the blending frequency and therefore the system stability and driver feel can be appropriately maintained over the whole of the vehicles operating conditions such as with vehicle speed where the high frequency gain can also be arranged to be a function of vehicle velocity.